Age-related macular degeneration (AMD) is the leading cause of blindness among people over the age of 50 in the western world. It is a bilateral, although asymmetric disease, and comes in two forms. Dry or non-neovascular AMD is the more common and milder form of AMD, accounting for 85-90% of all AMD. The key identifier for dry AMD is small, round, white-yellow lesions (also known as Drusen) in the macula. Vision loss associated with dry AMD is far less dramatic than in the case of wet AMD. There is currently no treatment available for dry AMD. It is estimated that as many as 14 million people suffer from dry AMD in the United States alone.
Wet AMD is less prevalent than the dry form, accounting for about 10-15% of AMD cases. The term “wet” denotes choroidal neovascularization (CNV), in which abnormal blood vessels develop beneath the retinal pigment epithelium (RPE) layer of the retina. Wet AMD is characterized by the development of choroidal angiogenesis which causes severe, and potentially rapid, visual deterioration. The visual distortion typically consists of perceiving straight lines as curved due to deformation of the retina in a region overlying the choroidal angiogenesis. The wet form of AMD accounts for about 60% of all cases of adult blindness in the United States. In the U.S. alone there are 200,000 new cases of wet AMD every year and a total of 1.7 million blind people from AMD.
Treatment modalities for wet AMD may include conventional treatments such as laser photocoagulation and newer treatment modalities such as Photodynamic therapy (PDT). Experimental treatments that are under current investigation include feeder vessel coagulation and trans-pupillary thermotherapy (TTT). All these proven or experimental therapies may halt or slow progression of the disease only if detected at an early stage and will not reverse existing retinal damage. Therefore, early detection is crucial to prevent severe visual loss.
Since approximately 12% of dry AMD cases develop wet AMD and subsequent blindness within 10 years, a patient diagnosed with dry AMD must be routinely examined by an ophthalmologist once or twice a year, depending on the severity of his condition. The patient is usually also given a so-called “Amsler grid” for weekly self-examination at home for symptoms of wet AMD. The patient is advised to consult an ophthalmologist immediately in the event that symptoms are noticed. The Amsler grid and its modifications (such as the “threshold Amsler” or the “red Amsler”) have been shown to be poor detectors of early changes associated with wet AMD for several reasons. One reason is the phenomenon of “filling-in” whereby the brain fills in missing parts in the pattern or corrects defects or distortions in the pattern. The subject thus fails to perceive a distorted pattern as being distorted. Another problem with the Amsler grid is the inability of patients to adequately fixate their vision on a fixed point while taking the test. The Amsler test also suffers from low compliance stemming from the non-interactive nature of the test.
The degree of visual deterioration is a function of the size of the lesion and its proximity to the fovea at the time of diagnosis. Although most lesions probably start outside the foveal area, 70% are already foveal and large (>1500 microns) at the time of diagnosis. It is therefore crucial to identify the lesions at the earliest possible stage, while they are still small and have not reached the fovea. It is known that 70% of lesions diagnosed as treatable become untreatable within less than three months, which indicates that the progression of the disease is relatively rapid. As many as 70-80% of patients with wet AMD are already ineligible for treatment when they first consult their ophthalmologist because the disease has progressed considerably. This is due to the poor validity of existing self-assessment methods for detecting an AMD-related lesion at an early stage, and the time lapsed between noticing the symptoms and seeing an ophthalmologist.
A reliable method for diagnosing wet AMD at the earliest possible stage, in conjunction with a referral system aimed at lowering the incidence of visual deterioration in this devastating disease, are imperative. If detected early, laser therapy to destroy the abnormal blood vessels may prevent additional vision loss. It is therefore crucial to detect the transition from dry to wet AMD as early as possible.